One of factors for letting an automobile safely travel includes pressure of a tire. When the pressure is decreased to less than a proper value, operating stability and fuel consumption are deteriorated, so that a tire burst may sometimes be caused. Therefore, a tire pressure monitoring system (TPMS) for detecting a decrease in tire pressure and issuing an alarm so as to urge a driver to take a proper action is an important technique from a view of protecting an environment and ensuring safety of the driver.
The conventional monitoring system can be classified into two types including a direct detection type and an indirect detection type. The direct detection type is to directly measure the pressure of the tire by incorporating pressure sensors inside a tire wheel. Although the decrease in the pressure can be detected with high precision, some disadvantages in terms of technique and cost remain including a need for a dedicated wheel and a problem in fault tolerance in an actual environment.
Meanwhile, the indirect detection type is a method for estimating the pressure from rotation information of the tire, and can further be classified into a DLR (Dynamic Loaded Radius) method and a RFM (Resonance Frequency Mechanism) method. The DLR method is a method for utilizing a phenomenon that a dynamic loaded radius is decreased due to crush of a deflated tire during traveling and as a result the tire is rotated more quickly than a tire of normal pressure, and comparing rotation speed of four tires so as to detect the decrease in the pressure. Since calculation processing is relatively easily performed with using only rotation speed signals of wheels obtained from wheel speed sensors, the DLR method has been widely studied for a purpose of mainly detecting a puncture of one wheel. However, it is anything more than relative comparison between the rotation speed of the wheels. Thus, a case where the four wheels are deflated at the same time (natural leakage) cannot be detected. Since a wheel speed difference is also generated by traveling conditions such as turning of the vehicle, acceleration/deceleration, and a biased load, there is a problem that deflation cannot be precisely detected through all the traveling states.
On the other hand, the RFM method is a method for utilizing a change in a frequency characteristic of wheel speed signals due to deflation so as to detect a difference from normal pressure. Unlike the DLR method, due to absolute comparison with preliminarily held normal values of wheels, the RFM method can respond to deflation of four wheels at the same time and is drawing attention as a better indirect detection method. The present invention relates to a tire state detection apparatus based on the RFM method. Hereinafter, the basic principle of this method will be described in detail.
When a vehicle travels, torsional motion in the front and rear direction emerging upon a tire receiving a force from a road surface and front-rear motion of suspension cause coupled resonance. This resonance phenomenon exerts an influence over rotation motion of a wheel. Thus, information on the resonance phenomenon is also included in a wheel speed signal acquired from a wheel sensor which is installed in an anti-lock braking system (ABS). Further, the coupled resonance is an inherent vibrating mode due to torsional rigidity of the tire. Thus, an excitation state thereof is changed depending only on a change in pressure constituting a physical characteristic of the tire, hardly depending on changes in vehicle speed and the road surface. That is, when the pressure is lowered, dynamics of the torsional motion of the tire are changed. Thus, when the wheel speed signal is frequency-analyzed, a peak made by the coupled resonance (hereinafter, referred to as the “resonance peak”) emerges on the lower frequency side at the time of deflation than the time of normal pressure. This phenomenon emerges independently from a type of the tire or the vehicle, traveling speed, a situation of the road surface, and the like due to the above-mentioned characteristic. Thus, the RFM method focuses on a resonance frequency, and issues an alarm in a case where the resonance frequency is relatively lower than a reference frequency estimated at the time of initialization. There is a need for estimating the resonance frequency from the wheel speed signal obtained from the ABS or the like which is installed in the vehicle. Regarding estimate of such a resonance frequency, various methods have been conventionally proposed (for example, refer to Patent Literature 1).
In the RFM method, in order to retain the resonance frequency with normal pressure of the tire, in a case where the tire pressure is adjusted or the tire is replaced, there is a need for procedure of initializing a monitoring system and after that storing a resonance frequency obtained by traveling for a fixed time as a reference frequency (leaning step). After learning is completed, the processing is automatically shifted to judgment by comparing the stored reference frequency and the current resonance frequency.